The decomposition of light into its component frequencies (wavelengths) is fundamental to optical spectroscopy. According to this invention a class of optical devices is provided that serve as configurable spectral filters. The devices receive an incident beam composed of light of different frequencies. They direct part of the light in a certain direction or onto a focal point. The devices consist of a system of movable diffractive microstructures on a substrate. By applying different voltages to the device, we alter the relative positions of the microstructures and thus we also alter the spectral composition of the diffracted light.
Spectral filters are important for all kinds of optical measurements. In the following we use the term spectral filter in a broad sense to describe all devices that selectively remove light with specific frequencies, totally or partially, from a lightpath. If the properties of the filter can be changed over time by applying different voltages, temperatures or other means of actuation we call it a tunable or configurable filter. There is no clear distinction between tunable and configurable except that the latter implies that a larger range of possible filter functions can be realized. Configurable optical filters are particularly important for spectroscopy. One example is the tiltable grating of a conventional monochromator. By our definitions this is an example of a tunable (or configurable) optical filter.
A diffractive optical element is a generalized 1-or 2-dimensional optical grating or hologram, made to synthesize a light field by exposing different parts of an incident light beam to different phase delays and/or amplitude modulations. With micro-electromechanical systems (MEMS) it is possible to make configurable DOEs. With current silicon micromachining technology it is not difficult to make moving parts with dimensions less than 10 micrometer. In a configurable DOE (CDOE) the top of such moving parts will be an optical surface: Mirrors, gratings or more complex structures for filtering or focusing. In the following we will call each surface a diffractive sub-element. Their relative positions can be tuned with a resolution much less than typical optical wavelengths, and the interference between reflections from the different parts determines the resulting light field.
A CDOE for synthesizing spectral filters (“The Polychromator”) has been designed and implemented by G. B. Hocker et al. “The polychromator: A programmable mems diffraction grating for synthetic spectra.” In Solid-State Sensor and Actuator Workshop, pages 89-91, Hilton Head Island, S.C., June 2000. This device is an electrostatically controlled array of beams. The beams can move vertically and the top of each beam is reflecting and plays the role as a grating element. The underlying theory of synthetic filters is given by Michael B. Sinclair et al. “Synthetic spectra: a tool for correlation spectroscopy.” Applied Optics, 36(15), 1997, which in turn is based on the phase retrieval algorithm (PRA) developed by Gerchberg and Saxton around 1970. (See for instance J. R. Fienup. “Phase retrieval algorithms: a comparison.” Applied Optics, 21(15): 58-69, 1982.) The disadvantage of “The Polychromator” is that light with a wide spectral range is diffracted at one single angle, limiting the resolution that can be obtained. Higher resolution can be obtained with a larger number of beams, but this increases the complexity of the device and makes it impractical to control. It is also a disadvantage that light cannot be focused onto a detector. This is possible with a concave ruled grating or, as we will show, a focusing diffractive pattern. Finally, the many gaps between the moving grating elements may reduce diffraction efficiency.
U.S. Pat. No. 5,905,571, Optical apparatus for forming correlation spectrometers and optical processors, describes an optical apparatus for forming correlation spectrometers and optical processors. The optical apparatus comprises one or more diffractive optical elements formed on a substrate for receiving light from a source and processing the incident light. The optical apparatus includes an addressing element for alternately addressing each diffractive optical element thereof to produce for one unit of time a first correlation with the incident light, and to produce for a different unit of time a second correlation with the incident light that is different from the first correlation. In preferred embodiments of the invention, the optical apparatus is in the form of a correlation spectrometer; and in other embodiments, the apparatus is in the form of an optical processor. In some embodiments, the optical apparatus comprises a plurality of diffractive optical elements on a common substrate for forming first and second gratings that alternately intercept the incident light for different units of time. In other embodiments, the optical apparatus includes an electrically-programmable diffraction grating that may be alternately switched between a plurality of grating states thereof for processing the incident light. The optical apparatus may be formed, at least in part, by a micromachining process.
U.S. Pat. No. 5,757,536, Electrically-programmable diffraction grating describes an electrically-programmable diffraction grating. The programmable grating includes a substrate having a plurality of electrodes formed thereon and a moveable grating element above each of the electrodes. The grating elements are electrostatically programmable to form a diffraction grating for diffracting an incident beam of light as it is reflected from the upper surfaces of the grating elements. The programmable diffraction grating, formed by a micromachining process, has applications for optical information processing (e.g. optical correlators and computers), for multiplexing and demultiplexing a plurality of light beams of different wavelengths (e.g. for optical fiber communications), and for forming spectrometers (e.g. correlation and scanning spectrometers). A device of this type has the disadvantage of having to control a large number of elements to achieve high spectral resolution within a narrow spectral range.